Alkaline secondary batteries typified by nickel-hydrogen batteries and nonaqueous electrolyte secondary batteries typified by lithium ion batteries have been widely used as a power supply for driving portable electronic equipment, such as cell phones including smartphones, portable personal computers, PDAs, and portable music players. In addition, alkaline secondary batteries and nonaqueous electrolyte secondary batteries have been widely used as a power supply for driving electric vehicles (EVs) and hybrid electric vehicles (HEVs and PHEVs), and in stationary storage battery systems for suppressing output fluctuation of solar power generation and wind power generation, for example, and for a peak shift of system power that utilizes the power during the daytime while saving the power during the nighttime.
The use of EVs, HEVs, and PHEVs or the stationary storage battery system especially requires high capacity and high output characteristics. The size of each battery is therefore increased, and a plurality of batteries are connected in series or in parallel for use. Therefore, nonaqueous electrolyte secondary batteries have been generally used for these purposes in view of space efficiency. When physical strength is needed, a metal bottomed prismatic hollow outer can with one side open, and a metal sealing plate for sealing this opening are generally adopted as an outer can of a battery.
Increasing longevity is essential in nonaqueous electrolyte secondary batteries used for the above-mentioned purposes. Therefore, various additives are added to a nonaqueous electrolyte in order to prevent degradation. For example, JP-A-2009-129541 discloses that, in a nonaqueous electrolyte secondary battery, a cyclic phosphazene compound and various salts having an oxalate complex as an anion are added to a nonaqueous electrolyte. JP-T-2010-531856 and JP-A-2010-108624 describe the addition of lithium bis(oxalato)borate (Li[B(C2O4)2], hereinafter referred to as “LiBOB”), which is a lithium salt having an oxalate complex as an anion, as represented by the following structural formula (I).

For example, Japanese Patent No. 3439085 discloses the invention of a nonaqueous electrolyte secondary battery in which lithium difluorophosphate (LiPF2O2) is added to a nonaqueous electrolyte in order to prevent self-discharge at charge storage and improve storage characteristics after charging. JP-A-2007-227367 shows an example in which LiPF2O2 is added to a nonaqueous electrolyte in order to obtain a nonaqueous electrolyte secondary battery having excellent cycling characteristics and low-temperature outputs.
When a cyclic phosphazene compound and various salts having an oxalate complex as an anion disclosed in JP-A-2009-129541 are added, fire resistance of the nonaqueous electrolyte is improved, which can provide a nonaqueous electrolyte secondary battery having excellent battery characteristics and high safety. When LiBOB disclosed in JP-T-2010-531856 and JP-A-2010-108624 is added to a nonaqueous electrolyte, a protective layer including a lithium ion conductive layer that is thin and extremely stable is formed on the surface of a carbon negative electrode active material of the nonaqueous electrolyte secondary battery. This protective layer is stable even in a high temperature, consequently preventing the carbon negative electrode active material from decomposing the nonaqueous electrolyte. This leads to an advantage of providing excellent cycling characteristics and improving the safety of a battery.
In the nonaqueous electrolyte secondary battery disclosed in Japanese Patent No. 3439085, LiPF2O2 and lithium are reacted in a nonaqueous electrolyte to form a high-quality protective covering onto an interface of a positive electrode active material and a negative electrode active material. This protective covering prevents direct contact between an active material in a state of charge and an organic solvent, thereby preventing decomposition of the nonaqueous electrolyte due to contact between the active material and the nonaqueous electrolyte. Consequently, an advantageous function effect of improving charge storage characteristics can be attained. In the nonaqueous electrolyte secondary battery disclosed in JP-A-2007-227367, a protective covering formed due to the LiPF2O2 brings preferable cycling characteristics and gives an advantageous effect of obtaining a nonaqueous electrolyte secondary battery that has excellent low temperature characteristics.
In a nonaqueous electrolyte secondary battery using a nonaqueous electrolyte in which a lithium salt having an oxalate complex as an anion is added to a nonaqueous solvent, a problem has been found that, when a battery is in an abnormal condition due to being crushed, for example, and the temperature thereof increased, the reaction is likely to proceed between a negative electrode formed with a protective covering and the nonaqueous electrolyte. This increases the amount of heat generation of the battery. A nonaqueous electrolyte secondary battery requiring high capacity and high output characteristics requires large absolute amounts of a negative electrode mixture and a lithium salt having an oxalate complex as an anion, which are responsible for the heat reaction.
Nonaqueous electrolyte secondary batteries can be used in a low temperature environment since EVs, HEVs, and PHVs are used outside. However, there is a problem in that a low temperature environment increases the viscosity of a nonaqueous electrolyte of the nonaqueous electrolyte secondary battery, thereby lowering output characteristics.